Engine mounting bolts

After I posted a crash picture of Mikey's Super Clown and totally destroyed LA 25, someone suggested using less strong mounting bolts on the engine. That way there would be a planned weak spot, as it were. I have had the cheap slot head sheet metal screws shear off in crashes with R/C planes using those plastic motor mounts. Old time free flight planes often had wood shear pins on the mounting beams, or plywood U-shaped mounts for upright motors.

Of course, it would be nice to continue using socket head bolts for ease of installation, especially under the exhaust stack. McMaster Carr has all kinds of socket head bolts in stainless, regular, high strength, even brass. I'm specifically looking at the 4-40 size. Microfasteners lists aluminum bolts with socket heads.

To get to the point: would changing the bolt material give any better crash resistance? I have bent stainless bolts; maybe something more brittle would be better. How tight does the mounting need to be? Or is it all just whistling in the dark when flying over asphalt? We looked at the divot last week, hard to believe wimpier bolts would have saved the engine. It's not like OS uses super alloy for the cases of their low-end motors.

I could go to nylon bolts; they make them in 4-40 socket head also. Just the thing to keep from crushing the wood on the front of the ARF Flite Streak. Is that what the R/C guys would call a "soft mount"?

My suggestion is "fly over grass." That has been my solution for the last 49 years...............it works well........I have never destroyed an engine in a crash, and I have had a few crashes over the years...............T.Durrill

The cost of the materials in the plane, and especially the time invested, far outweigh the cost of the engine. While it's always a minor tragedy to lose a good engine, I'll stick with the added security of case-hardened socket head screws.

There's no guarantee that weaker screws would save the engine in a crash, anyhow, especially on concrete. If the engine dies, it dies.

Too fatalistic ? ? ?

This was given to me by someone else who flies on pavement.

Your best bet is probably plain old soft steel machine screws- Sig, Tower, MicroFasteners. the head will break off if you try to torque them too hard. The screws also have a tendency to break in a crash, but they are proven strong enough to handle up to 40 engines pretty reliably and are very cheap. Probably not much different than brass in strength.

"Or is it all just whistling in the dark when flying over asphalt?"

Yepper..Just look at it this way,,,,Take your engine, stand in the middle of the parkin' lot, throw it fifty feet in the air, and see what happens when it "lawn darts". Ugly aint it? Some times bad things happens to good engines.... Say some kind words, stick it in the ground, have a wake, get a new one, get on down the road....

Life's to short to worry.

It's no big deal to swap in another motor. A couple of runs and these modern Asian motors seem to be broken in enough to fly. Not like some made in that former commie place Reagan called "The Great Satan." Just the cost of playing with our toys. I quit using the old baffle piston motors because parts were too hard to find. The OS engines are available most everywhere, if you don't mind changing from R/C carb to venturi.

I might try some cheapy hardware store or Sig plain steel bolts. To avoid the problem of screwdriver access under the exhaustturn them around to put the heads on the side of the fuselage opposite the motor and use nuts on top of the lugs. That way they can be held with a spanner or box wrench and the bolts tightened from the other side with a screwdriver. Now where is my Sears ignition wrench set? Ever try to buy a 1/4 inch combo wrench? Gotta get one of those tiny Crescent wrenches like Smoothie Bill has.

Just as the others have suggested, you really need to fly over, or at least crash in, grass.